There are the odd exceptions to the rule with ERST's in regards to chatter. I know of a couple that do chatter all day long, even at jsut 9/10psi of boost. The only explantion is that the metering unit (between turbo and air filter is on the way out and not working rightly. Don't hold me to this, as it is not proven, but when they are doing what they should be doing, with the throttle shut (ie: your foot off pedal) the metering unit should be shut too. Inside the metering unit is a big round plate on a pivoting arm. Air being sucked in from the air filter pushes the arm down, thus making the plate go down too. When charge air has to find it's own way out with throttle snapped shut, it naturally goes back through the charge system. But it should not be able to escape thought the metering unit as the plate only opens one way and the escaping charge air will be traveling backwards. The only thing it can do is push harder against the plate, and pushing from under the plate will definitely close it.

So to creat chatter you have to have air pass through the turbo and out the air filter for you to hear it. It is not possible for this air, once it has gone back through the turbo to escape via the metering unit, if the metering unit is working properly. If air IS getting out past the metering "flap" then there must be something wrong with the metering unit for air to escape from it.

If you get what I mean